In an arm pivoting about an axis of rotation, the torque, with respect to the axis of rotation, due to the weight of the arm and any fixture supported by it, often varies alongside the angular position of the arm about the axis. The arm can be used for various types of equipment, particularly those involving moving the fixture by manually rotating the arm.
To reduce the effort required of the user, the torque due to the weight of the arm is normally balanced by elastic means, such as a contrast spring; and, to compensate the variation in torque produced by a variation in the angular position of the arm, means have been proposed for automatically controlling the action of the spring according to the angular position of the arm.
In an all-purpose fume exhaust conduit, an arm balancing device has been proposed in which the action of the spring is regulated by means fitted to an articulated parallelogram, the pivot of which is eccentric with respect to the axis of rotation of the arm. Though satisfactory in terms of performance, the above balancing device is relatively complex and expensive to produce, on account of the large number of moving parts, and the rods of the parallelogram having to be formed to house the spring inside a substantially closed channel.